High voltage insulator with internal corona shield



May'Z, 1967 HIGH VOLTAGE INSULATOR WITH INTERNAL CORONA SHIELD FiledSept. 22, 1965 3 Sheets-Sheet l H. A. FREY 3,317,659

y 2, 1967 H. A. FREY 3,317,659

I HIGH VOLTAGE INSULATOR WITH INTERNAL CORONA SHIELD Filed Sept. 22,1965 I 3 heets-Sheet 2 May 2, 1967 H. A. FREY 3,317,659

HIGH VOLTAGE INSULATOR WITH INTERNAL CORONA SHIELD 17/3 Mir/75y.

United States Patent 3,317,659 HIGH VOLTAGE INSULATOR WITH INTERNALCORONA SHIELD Howard A. Frey, Towson, Md., assignor to General ElectricCompany, a corporation of New York Filed Sept. 22, 1965, Ser. No.489,235 2 Claims. (Cl. 174-140) This invention relates to electricalinsulators and more particularly to improvements in corona shields forthe end hardware of high potential power line insulators.

Such insulators, of either the strain or suspension type on the one handor the post or column type on the other hand, usually have an end atground potential and the other end at line potential. At the higher linepotentials the potential gradient in the air adjacent the line endhardware fittings becomes high enough to cause corona discharge which isa high frequency disturbance disruptive of adjacent communicationchannels and therefore highly objectionable. Accordingly, it has beenconventional to equip such line end hardware fittings with externalcorona shields which in general have smoothly contoured comparativelylarge conductive surfaces at line potential in proximity to the line endhardware for reducing the potential stress concentration in the airadjacent the line end hardware.

In accordance with this invention, '1 have found that similar resultscan be obtained by embedding an enlarged smoothly contoured conductivesurface in the insulating material of the insulators adjacent the linehardware. Such embedded surface may either be c-onductively coupled orcapacitively coupled to the line end hardware. In either case itincreases the effective surface area of the line end hardware and thusconstitutes an internal corona shield which reduces the potentialgradient in the air surrounding the line end hardware.

Such an internal corona shield has numerous advantages 'overconventional external corona shields. Thus it is less costly, does notrequire separate installation as it is an integral built-in part of theinsulator and also does not increase the size of the line end of theinsulator so that icing and windage forces on the insulator are reduced.

Corona shields, in reducing the potential stress or gradient at the lineend of an insulator, also inherently tend to grade the voltage stressalong the insulator. In accordance with another feature of thisinvention, additional or auxiliary conductive surfaces may be embeddedin the insulator which in cooperation with main internal coronashielding surface further grade the potential stress distribution alongthe insulator and thus enhances the corona shielding action of the maininternal shielding surface.

An object of the invention is to provide a new and improved electricalinsulator.

Another object of the invention is to provide an internal corona shieldfor a high voltage insulator.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings and its scope will bepointed out in the appended claims.

In the drawings,

FIG. 1 is a vertical cross sectional view of a pin type insulatorembodying the invention,

FIGS. 2, 3, and 4 'are similar views of modifications of FIG. 1,

FIG. 5 is a similar view of a post type insulator embodying theinvention,

FIG. 6 is a similar view of a modification of FIG. 5,

FIG. 7 is a similar view of a suspension type insulator embodying theinvention,

FIG. 8 is a similar view of a modified post type insulator having anembedded resistor, and

3,317,659 Patented May 2, 1967 FIG. 9 is a similar view of a column typeinsulator in which a number of conducting bubbles are provided forshielding and grading purposes.

Referring now to the drawings and more particularly to FIG. 1, there isshown therein an insulator of what is commonly called the pin type. Ithas a body 1 formed in any suitable manner, such as by molding, ofinsulating material of any suitable type such as porcelain or polymericmaterial. As shown it is provided with integral shields, petticoats, orrainhoods 2 and a metallic mounting pin 3, the upper surface including agroove or saddle 4 for receiving a high voltage line conductor 5 andside grooves or saddles 6 for receiving a tie wire 7 for fastening theline conductor 5 to the top of the insulator. A conducting film 8 isencapsulated in the insulating material. As shown, it is generallyrounded and generally conforms to the adjacent shape of the highpotential conducting parts of the assembly, namely the line conductor 5and the tie wire 7, but is of substantially greater extent or surfacearea than those parts. The conducting film, therefore, reduces theelectric stress in the air around the conductor 5 and tie wire 6 inorder to raise the corona starting voltage and so eliminate radio ortelevision interference at operating voltage. The conducting film mayconsist of foil, of formed sheet metal, of wire mesh or of a conductingfilm of high resistance material such as that known to the trade asRescon whose principal constituent is carbon black. As shown in FIG. 1,the conducting film 8 is completely insulated from the conductor 5 andtie wire 7 so that it floats at a potential near that of those partsbecause of its capacitance. Consequently, this comparatively large,comparatively smooth contoured surface at a potential substantially thatof the line conductor 5 and tie wire 7 serves to reduce the potentialgradient in the air surrounding the parts 5 and 6 below what it would beif the film 8 were not present.

In the modification shown in FIG. 2, the conducting corona shieldingfilm 8 is brought to the surface of the insulating material 1 at thebottom of the grooves 4 and 6 so that the line conductor 5 and tie wire7 may make direct mechanical and electrical contact therewith. In thatcase, of course, the parts 5, 7 and 8 are all at the same operatingpotential. V

In the modification shown in FIG. 3, the body of insulating material 1has a metallic cap 9 rather than grooves 4 and 6 at its top. Such aninsulator is suitable for supporting bus bars and other conductors. Itis provided with a corona shielding conductive film 8 corresponding tothe film in FIG. 1 in that it is closely adjacent to and conforming ingeneral shape to the extended surface of the cap 9 but it is notconnected thereto. For further increasing. the corona starting voltageof the insulator by incidentally grading the potential distributionthrough the insulation between the parts 9 and 3 additional spacedconductive films 10 and 11 are shown.

FIG. 4 is generally similar to FIG. 3 except that the main coronashielding embedded film is a ring 8" which is electrically connected atits inner edge to the outer edge of the cap 9. In this manner, theeffective surface of cap 9 is increased and the voltage at which coronawill appear at its outer edge is materially increased.

FIG. 5 illustrates a post type insulator having a central body ofinsulating material 12 provided with petticoats 13 and with metallic endcaps 14, the bottom. one being normally at ground potential and theupper one being :at line potential or at some other potential which isrelatively great compared to ground. Embedded in the insulation 12 is aconductive film 15 of rounded contour following generally the shape ofthe underside of the cap 14 when extended for acting as a corona shieldfor the top cap 14. Additional potential grading films 16, 17, 18 and 19may also be embedded in the insulation so that the capacitancestherebetween will serve to reduce the unit stress in the insulation andhence in the surrounding air adjacent the upper cap 15.

FIG. 6 is generally the same as FIG. showing a somewhat modified type ofcolumn or post insulator in which the primed reference numberscorrespond generally to the reference numbers in FIG. 5 but are ofsomewhat different shape.

FIG. 7 is a cross sectional view of a suspension type insulator unit inwhich a main disc like body of insulation has embedded therein onopposite sides a metallic ball socket 21 and ball pin 22. Acting as amain corona shield for the ball socket member 21 is a corona shieldingfilm 23 embedded in the insulation 20 and connected to the part 21.Similarly shielding the ball pin 22 is :a conductive film 24 embedded inthe insulation and connected to the part 22. An auxiliary potentialgrading film is also shown between the films 23 and 24.

It is believed that this type of construction is particularlly suitablefor so-cal led fog type insulators which are required to operate incontaminated atmospheres where surface dirt and moisture on theinsulators frequently badly distort the surface electric gradients.

In the modification shown in FIG. 8, the insulator 24 with its rainhoods25 and metallic end hardware fittings 26 and 27 has embedded therein ahigh resistance element 28 extending between the hardware fittings 26and 27 and connected respectively thereto at its ends. Connected atintermediate points on this high resistance element are conducting films29, 30 and 31 which serve to control the corona starting voltage of theend hardware fittings 26 and also the potential distribution in theinsulation 24, the potential difference between the film-s 29, 30 and 31being determined primarily by the resistance potential divider effect ofthe resistor 28 rather-than the capacitance coupling of the surfaces 29,30 and 31. The resistor 28 may consist of an embedded Carborundumresistor or it may consist of tapes or bands of material coated withRescon or with similarly conductive material such as carbon black. Itmay be straight or zigzag as shown and in the latter case may consist ofa very fine high resistance wire as shown.

In FIG. 9 is shown a column type insulator consisting of a main part ofinsulation 32 provided with petticoats 33 and hardware fittings or caps34 and 35 and internal hollow conductive bubbles 36, 37, 38 and 39 whichare devoid of solid insulating material. They may consist of hollowcanisters made of foil, sheet metal, or other conductive material. Theirsize, shape and placement may be varied to obtain varying internalcapacitance and so control gradient. Their ends are shown rounded orspherical in order to obtain the effect of a multiple sphere gap throughthe insulator. This construction has the further advantage of displacingexpensive insulating material and reducing overall weight of theinsulator.

In all of the illustrated species except FIG. 9, the conductive filmsmay be made of perforated metal or of wire mesh so that the mechanicalstress may be transmitted by the insulating material through the filmwithout producing a fault plane. The conductive films may also consistof a system of radial or circumferential bands instead of solid areas.They may further be made in total or in part of high resistance materialto gain further control of gradient.

As an example of the latter, the insulated material may be made of somehigh resistance material such as Rescon or of tapes, cloth, or fibercoated with Rescon.

It is also possible in any one particular film to make part of the filmof solid conducting material, such as foil, with other parts made ofhigh resistance material such as Rescon in order to gain further controlover gradient in this particular area. For example, the top and bottomconducting films shown in FIG. 7 might consist of an outer conductingring with the inner part of the conducting film made of Rescon or otherhigh resistance material so that the radial gradient outward could befurther controlled by the resistance of the inner high resistanceportions of the conducting zone. Further, in species like that of FIG.1, the outer periphery of the conducting film might be made of highresistance material to grade off the stress at the edges.

While there have been shown and described particular embodiments of theinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention, andtherefore it is intended by the appended claims to cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electrical insulator comprising a solid piece of electricalinsulation having external metallic parts at opposite ends between whichunder operating conditions a large difference in electrical potentialexists, and an integral internal corona shield for at least one of thoseparts comprising a conductor film embedded in said piece of insulationadjacent said one part, said film having a surface substantially largerthan but conforming in general shape to the adjacent surface extended ofsaid one part, said surface being reentrant and enclosing a space devoidof said solid insulation.

2. An electrical insulator comprising a solid piece of electricalinsulation having external metallic parts at opposite ends between whichunder operating conditions a large difference in electrical potentialexists, a first integral internal corona shield for at least one ofthose parts comprising a conductor film embedded in said piece ofinsulation adjacent said one part, said film having a surfacesubstantially larger than but conforming in general shape to theadjacent surface extended of said one part, at least one auxiliarycorona shielding conductive film is embedded in said insulator materialbetween the first shield and the other external metallic part foradditionally shielding said one part by incidentally grading thepotential stress distribution in said insulator between said parts, bothcorona shielding films being curved reentrant surfaces enclosing spacesdevoid of said solid insulation.

References Cited by the Examiner MD? A K Prima y Exam ner,

1. AN ELECTRICAL INSULATOR COMPRISING A SOLID PIECE OF ELECTRICALINSULATION HAVING EXTERNAL METALLIC PARTS AT OPPOSITE ENDS BETWEEN WHICHUNDER OPERATING CONDITIONS A LARGE DIFFERENCE IN ELECTRICAL POTENTIALEXISTS, AND AN INTEGRAL INTERNAL CORONA SHIELD FOR AT LEAST ONE OF THOSEPARTS COMPRISING A CONDUCTOR FILM EMBEDDED IN SAID PIECE OF INSULATIONADJACENT SAID ONE PART, SAID FILM HAVING A SURFACE SUBSTANTIALLY LARGERTHAN BUT CONFORMING IN GENERAL SHAPE TO THE ADJACENT SURFACE EXTENDED OFSAID ONE PART, SAID SURFACE BEING REENTRANT AND ENCLOSING A SPACE DEVOIDOF SAID SOLID INSULATION.